Dysfunction, injury, and loss of retinal pigment epithelium (RPE) cells are prominent features of certain eye diseases and disorders, such as age-related macular degeneration (AMD), hereditary macular degenerations including Best disease (the early onset form of vitelliform macular dystrophy), and subtypes of retinitis pigmentosa (RP). A potential treatment for such diseases is the transplantation of RPE (and photoreceptors) into the retina of those affected with the diseases. It is believed that replenishment of RPE cells by their transplantation may delay, halt or reverse degeneration, improve retinal function and prevent blindness stemming from such conditions.
The macula, the central part of the retina, is responsible for fine visual detail and color perception, and is crucial for many of our daily visual tasks such as facial recognition and reading. The macula is often affected as part of the disease process in widespread retinal degenerations such as retinitis pigmentosa (RP), as well as in different diseases that more specifically target the macular region such as age-related macular degeneration (AMD) and Best disease. In many of these diseases, the primary dysfunction and failure occurs in the retinal pigment epithelium (RPE) cells which underlie the photoreceptors.
The highly specialized RPE cells play a major role in supporting photoreceptor function: they actively transport nutrients from the choroidal vessels, participate in the recycling of vitamin A, which is necessary for the chromophores in the photoreceptors, and take-up and recycle shed photoreceptor outer segments as part of the normal renewal process of these cells1.
In subtypes of RP, Best disease, and AMD, failure of the RPE ultimately leads to visual loss and blindness. Replacement of these cells is a possible therapeutic intervention2, but obtaining such cells from human donors or embryos is difficult. Human embryonic stem cells (hESCs) may serve as a potential unlimited donor source for RPE cells, if the means to direct their differentiation into functional RPE cells can be elucidated3. Methods to direct the differentiation of hESCs into cultures highly enriched for neural precursor cells (NPs) have previously been described (Reubinoff B E. et al., Neural progenitors from human embryonic stem cells; Nat Biotechnol 19: 1134-1140, 2001; Itsykson P. et al., Derivation of neural precursors from human embryonic stem cells in the presence of noggin; Mol Cell Neurosci. 30(1):24-36, 2005). In addition, the potential of hESCs to give rise to retinal cells both in vitro and in vivo following transplantation to the subretinal space in rodents has been shown (Banin E. et al., Retinal Incorporation and Differentiation of Neural Precursors Derived from Human Embryonic Stem Cells; Stem Cells 24(2):246-257, 2006.)
The potential of mouse and non-human primate ESCs to differentiate into RPE cells, and to survive and attenuate retinal degeneration after transplantation, has been demonstrated4,5. Spontaneous differentiation of hESCs into RPE cells was shown, however, the efficiency of the differentiation process was low, a substantial time of differentiation was required and only a low (<1%) percentage of clusters containing RPE cells were obtained after 4-8 weeks of differentiation. Furthermore, while improved retinal function was observed in RCS rats after sub retinal transplantation of these RPE cells, function of the transplanted cells as authentic mature RPE cells was not demonstrated and this effect could potentially be related to a non-RPE-specific trophic effect.6,7,9,10.
It was also recently shown that hESCs may be directed to reproducibly differentiate into RPE cells, in which directed rather than spontaneous differentiation of hESCs towards an RPE fate occurred in the presence of Nicotinamide (NA)8.